The spectra resulting from transitions of optically oriented atoms in the presence of an electromagnetic field are known to be affected by a magnetic field so as to enable precise measurements of the magnetic field strength. A quantitative analysis of magnetic resonance in atomic spin systems may be found in many books and other publications. A brief qualitative explanation will suffice, however, for an understanding of the present invention.
Electrons are known to have a spin which gives rise to an intrinsic magnetic moment for the electron. If the electron is placed in an external magnetic field its magnetic moment interacts with the field to subject the electron to a torque. The electron, therefore, is caused to precess about the magnetic field at a rate proportional to the magnetic field strength.
Spectrographic observations have shown that an electron may assume only two discrete energy levels as a result of interaction between its spin magnetic moment and the external magnetic field. If a guantum of energy at a particular frequency, known as the resonance frequency, is supplied to electrons with one orientation some electron spin vectors can flip and assume the opposite orientation relative to the magnetic field. The resonance frequency is equal to the classical precession frequency.
Where the electrons are part of an atom, their movements are more complex due to the coupling of the electron spins to the internal magnetic fields of the atom, but the general qualitative behavior of an electron in an atom is very similar to that of a free electron. From a practical point of view, electrons in an atom are far more convenient for magnetic resonance applications than free electrons, since the atom as a whole is uncharged and has an indefinite lifetime under normal conditions. In contrast, a free electron has one unit of negative charge and it therefore interacts strongly with external electric and magnetic fields. Also, since free electrons rapidly recombine with positive ions, atoms, molecules or solid surfaces, the lifetime of a free electron is very short under normal conditions. It has been found that the atoms of alkali metal vapors are particularly convenient for use in systems designed for detection for magnetic resonance in optically pumped atoms.
One such system has been described by H. G. DEHMELT in U.S. Pat. No. 3,267,360, for use in a magnetometer. This patent relates to an optical technique, known as optical pumping, for aligning alkali atoms in an external magnetic field to be measured by the instrument. For alkali metals, photon energies corresponding to transitions between the ground state and the first excited state correspond substantially to optical wavelengths. Thus, alkali metal vapors are desirable for use in devices which utilize optical pumping techniques. The alignment of the alkali vapors is monitored by one of a number of techniques, for instance by detection of the light used to orient the alkali vapors into a "pumped" condition. As is well known in the art, monitoring of optically oriented alkali-metal vapors provides information from which the field strength of the external magnetic field may be calculated.
In the presence of a magnetic field, the total atomic angular momentum which results from both nuclear and electronic angular momenta, may precess about the field at discrete precession frequencies identifiable through appropriate monitoring of the atoms. The energy levels associated with these precession frequencies are known as Zeeman sublevels, and the totality of all Zeeman sublevels is called the hyperfine structure. Such hyperfine splitting of atomic energy levels occurs both in the ground-state and in the optically excited states.
When an atom has been excited, for example by absorbing a quantum of optical energy, from its ground state to an excited state, it decays through the various Zeeman sublevels back to the ground-state.
As disclosed in U.S. Pat. No. 3,267,360, the optical system which supplies light energy to the alkali atoms is designed to change the spin alignment of optically excited electrons. This may, for instance, be done through the use of a circular light polarizer disposed between the light source and the alkali atoms. When alkali atoms are illuminated by circularly polarized resonance light, the absorption rate of atoms in the ground-state sublevels corresponding to spin-down electrons is different from that of atoms in the ground-state sublevels corresponding to spin-up electrons. If no spin polarization is produced by the repopulation of the atomic ground state following spontaneous decay of the optically excited atoms, it is clear that more electrons will tend to accumulate in the weakly absorbing ground-state sublevels, compared to the strongly ground-state sublevels. Thus, over a period of time, usually on the order of 10.sup..sup.-3 seconds, the alkali atoms can become appreciably less absorbing of light from the light source, and they thus become more transparent to the light.
In accordance with the known techniques, the nonabsorbing electrons may be stimulated into a light absorbing sublevel by the application of an electromagnetic field at the magnetic resonance frequency for the electrons. At the resonance frequency, the electrons in the nonabsorbing sublevel of the ground state can make a transition to a light absorbing sublevel. When this occurs, the electrons absorb light, and therefore, the intensity of the pumping light which passes through the alkali material suddenly decreases. Once the resonance frequency for the electrons is determined by observing the intensity minimum, it is relatively simple to calculate the field strength of the external magnetic field acting on the system. It should be understood that it is also possible to detect magnetic resonance by observing the flourescence light from the vapor. In the case of flourescence light detection, magnetic resonance is indicated by an intensity maximum.
A comprehensive discussion of the quantum principles which underlie the operation of magnetometer may be found in an article by Robert E. Slocum and Francis N. Reilly entitled "Low Field Helium Magnetometer", IEEE Transactions on Nuclear Science, January 1963, pages 165 through 171. Also pertinent is an article by Arnold Green and J. M. Stanley, entitled "The Application of the Technique of Optical Pumping for the Precise Measurement of Weak Magnetic Fields", the Australian Physicist, March 1973, pages 52 through 54.
Devices which utilize these principles, however, have heretofore been thought to be subject to certain physical constraints, which severely limit their utility. (See, for example, Arnold L. Bloom, "Principles of Operation of the Rubidium Vapor Magnetometer" Applied Optics Vol. 1, No. 1, January 1962 pp. 64 and 65; and J. H. Allen, P. L. Bender "Narrow Line Rubidium Magnetometer for High Accuracy Field Measurements" J. Geomag. Geolectr., 24, 105-125, 1972) It has been thought heretofore that low density alkali vapor regimes were required to ensure accurate detection of the resonance frequency. Spectroscopic experiments on the effect of spin exchange collisions on linewidths and intensities of electromagnetic transitions of optically oriented atoms have shown a broadening of the width of the magnetic resonance curve with increasing density. Such broadening creates uncertainty in measurement of the resonance peak. As a result magnetometer devices heretofore have utilized relatively low density alkali-vapor regimes. This has required magnetometers to be relatively large and bulky, generally having an absorption cell volume of approximately 100 cubic centimeters. Such equipment, therefore, has been expensive and is, of necessity, limited to minimum risk uses. In addition, since the number of atoms to be pumped is relatively low the light intensity must be correspondingly low, usually about 10.sup..sup.-3 watts. The result has been excessive shot noise in comparison to the signal which severely limits the sensitivity of the equipment.